Transformer: Fix bug where reflection button appeared off the line
Transformer: Fix bug where reflection button appeared off the line

Summary:
Fixes a small but annoying bug where the reflection button could end up
off of the reflection line if you moved the line quickly enough.
This was caused because onMoveEnd sometimes gets called from a
movablePoint without onMove() being called on those same coordinates
first.

Test Plan:
Try to get the center button to lag off the line by moving around one of
the line of reflection's points quickly and then releasing it while
moving.

Auditors: alex

ariabuckles/perseus,daukantas/perseus,alexristich/perseus,junyiacademy/perseus,ariabuckles/perseus,learningequality/perseus,huangins/perseus,ariabuckles/perseus,chirilo/perseus,abe4mvp/perseus,alexristich/perseus,abe4mvp/perseus,learningequality/perseus,iamchenxin/perseus,iamchenxin/perseus,daukantas/perseus,junyiacademy/perseus,chirilo/perseus,huangins/perseus,ariabuckles/perseus,sachgits/perseus,sachgits/perseus